1. Field of the Invention
This invention relates to a method of enhancing sterilization with a sterilant vapor and plasma with improved material compatibility.
2. Description of the Related Art
Some new commercial systems for sterilizing medical instruments and the like utilize low-temperature reactive gas plasma to achieve rapid, low-temperature, low-moisture sterilization of medical items. Low-temperature gas plasma is sometimes described as a reactive cloud which may contain ions, electrons, and/or neutral atomic particles. This state of matter can be produced through the action of electric or magnetic fields, or through other external forces such as high-energy particle flux. In general, an electric field can be in any frequency range (an example of a naturally occurring plasma is the aurora borealis or the northern lights). One commercial embodiment of plasma sterilization is the STERRAD(copyright) Sterilization Process, as described in U.S. Pat. No. 4,643,876.
The STERRAD Sterilization Process is performed in the following manner. The items to be sterilized are placed in the sterilization chamber, the chamber is closed, and a vacuum is drawn. An aqueous solution of hydrogen peroxide is injected and vaporized into the chamber so that it surrounds the items to be sterilized. After reduction of the pressure in the sterilization chamber, a low-temperature gas plasma is initiated by applying radio frequency energy to create an electrical field. In the plasma, the hydrogen peroxide vapor is dissociated into reactive species that collide/react with and kill microorganisms. After the activated components react with the organisms or with each other, they lose their high energy and recombine to form oxygen, water, and other nontoxic byproducts. The plasma is maintained for a sufficient time to achieve sterilization and remove residuals. At the completion of the process, the RF energy is turned off, the vacuum is released, and the chamber is returned to atmospheric pressure by the introduction of High Efficiency Particulate-Filtered Air (HEPA).
The above-described sterilization system can safely process medical items currently sterilized by ethylene oxide and steam, with the exception of linens and other cellulosic materials, powders, and liquids. Sterilized items arc ready to be used in a little over an hour after starting the sterilizer. The process requires no aeration, and there are no toxic residues or emissions. Preparation of instruments for sterilization is similar to current practices: cleaning the instruments, reassembly, and wrapping. The system typically uses non-woven polypropylene wraps or sterilization pouches made of at least one permeable side, both of which are commercially available, and a tray and container system. A special vessel containing liquid sterilant can be placed on long, narrow lumen instruments to allow rapid sterilization of their channels. A chemical indicator specifically formulated for this process is used, as well as a specifically designed biological indicator test pack.
The efficacy of the STERRAD Plasma sterilization system has been demonstrated. Depending upon the particular design, plasma sterilization systems can therefore provide efficient, safe methods for sterilizing medical instruments and other hospital products.
For optimum operation, a plasma sterilization system such as that described above requires the loads that are to be sterilized to be quite dry. However, normal hospital practice in the preparation of instruments for sterilization often results in levels of water that may be excessive. The excess water makes it difficult to achieve the low-pressure thresholds required to initiate the sterilization process. To initiate the sterilization process, the chamber pressure is preferably reduced to relatively low levels, for example approximately 200-700 mTorr. Since the equilibrium vapor pressure of water is significantly higher than 700 mTorr at room temperature, any water in the chamber or load will begin to vaporize during the vacuum phase. The heat of vaporization required for the water to vaporize causes the load and any remaining water to chill. When enough water has vaporized, the remaining liquid begins to freeze. Eventually, the remaining liquid will completely freeze, which slows the rate of vapor generation and retards the attainment of the pressure levels required for optimum operation of the sterilizer. These conditions can cause undesirably long sterilization cycles or even cancellation of the sterilization cycle. Spencer et al. (U.S. Pat. No. 5,656,238) disclosed that plasma can be used to enhance the drying so that the desired pressure for sterilization may be achieved more quickly.
Improper plasma treatment can lead to damage to materials in the chamber or in the equipment, however. There is a need for a method of enhancing material compatibility while simultaneously achieving high sterilization efficiency.
One aspect of the invention involves a method of sterilizing articles in a load in a chamber with a chemical sterilant. The method includes conditioning the load, then introducing chemical sterilant; and maintaining to achieve sterilization. Conditioning the load includes evacuating the chamber, generating plasma in the chamber, venting the chamber to approximately atmospheric or subatmospheric pressure, and repeating the evacuating, generating plasma, and venting at least two times.
Preferably, conditioning the load includes increasing the temperature of at least a portion of the load to at least 30xc2x0 C. Advantageously, conditioning the load comprises increasing the temperature of at least a portion of the load to at least 35xc2x0 C. In a preferred embodiment, the chemical sterilant is hydrogen peroxide.
Advantageously, plasma is generated in the chamber when the sterilant is introduced or during the maintaining. Preferably, the method also includes venting the chamber to a pressure, maintaining the pressure, and then evacuating the chamber, where the venting is after the maintaining. Advantageously, the plasma generated during the introducing of the sterilant or the maintaining is generated with lower power than the plasma generated after conditioning and evacuating.
Another aspect of the invention involves a method of reducing sterilant residuals on articles in a load in a chamber. The method includes evacuating the chamber a first time, introducing sterilant, maintaining to achieve sterilization, venting the chamber to a pressure, maintaining the pressure, evacuating the chamber a second time, venting the chamber a second time, and removing the articles in the load from the chamber.
Advantageously, the venting pressure is atmospheric or sub-atmospheric pressure. Preferably, plasma is generated in the chamber during the introducing of the sterilant, during maintaining, or evacuating a second time.
Advantageously, the venting, maintaining, and evacuating a second time are repeated. Preferably, the chamber is evacuated, plasma is generated, and the chamber is vented before the method of reducing process residuals is carried out.
Yet another aspect of the invention involves a method for sterilizing devices in a chamber, where the method includes at least two plasma steps, where at least one plasma step occurs before introducing the chemical sterilant and at least one plasma step occurs after introducing the chemical sterilant. The method includes generating plasma with a higher power level in the plasma step occurring before the chemical sterilant is introduced than in the plasma step occurring after the chemical sterilant is introduced.
Preferably, the chemical sterilant is hydrogen peroxide. Advantageously, the method also includes venting the chamber and evacuating the chamber after generating plasma with the higher power level.
In an embodiment, the method also includes venting the chamber to a pressure, maintaining the pressure, and evacuating the chamber, where the venting, maintaining, and evacuating occur after the plasma step occurring after the chemical sterilant is introduced.
Another aspect of the invention involves a method of sterilizing articles in a load with a chemical sterilant in a chamber with improved material compatibility. The method involves evacuating the chamber, generating plasma with a first power level, venting the chamber to a pressure, evacuating the chamber, and introducing chemical sterilant into the chamber. Introducing the sterilant occurs after generating plasma with the first power level. The chamber is evacuated, plasma with a second power level is generated, where the plasma with the second power level is generated after the sterilant is introduced. The method also includes venting the chamber, where the venting occurs after generating plasma with the second power level. The chamber is then evacuated and vented. The first power level is higher than the second power level, thereby sterilizing the articles with improved material compatibility. Advantageously, the chemical sterilant is an antimicrobial agent. Preferably, the antimicrobial agent is hydrogen peroxide.
Advantageously, evacuating, generating plasma with the first power level, and venting is repeated more than once. Preferably, the chamber is venting after the chemical sterilant is introduced into the chamber Advantageously, the pressure is maintained after venting. In an embodiment, additional plasma is generated in the chamber after generating plasma with the second power level, venting, and evacuating. Advantageously, the venting and evacuating are repeated.